This invention relates generally to apparatus for data transfer with a rotating data storage disk such as, typically, a flexible magnetic disk of three and a half inch diameter packaged in cassette form. More specifically, the invention pertains, in such rotating disk data storage apparatus, to a novel system for generating index pulses indicative of the angular position and rotational speed of the disk.
The three and a half inch flexible magnetic disk and a disk drive for use therewith are both described and claimed in U.S. Pat. No. 4,445,157 to Takahashi. The magnetic disk is rotatably housed in a generally flat, boxlike envelope of relatively rigid plastic material, complete with a metal made sliding shutter, to make up a disk cassette. The envelope has a pair of apertures of rectangular shape to expose radial portions of the opposite sides of the magnetic disk. The sliding shutter also has a pair of apertures of approximately the same shape and size as the envelope apertures, which shutter apertures come into and out of register with the envelope apertures. The magnetic disk has a rigid hub of magnetic sheet metal attached centrally thereto. The hub has defined therein a central opening of square shape and an eccentric opening of rectangular shape.
When positioned in the associated disk drive, the magnetic disk has its central hub placed on a turntable which is much less in diameter than the disk. The turntable has a permanent magnet mounted thereon for attracting the disk hub. Disposed centrally on the turntable, a spindle engages in the central opening in the disk hub for centering the magnetic disk on the turntable. A drive pin is disposed eccentrically on the turntable for driving engagement in the eccentric opening or slot in the disk hub, imparting the rotation of the turntable to the magnetic disk.
The eccentric slot in the disk hub has a predetermined positional relationship to a reference position (e.g. starting position of the first sector) on the disk. Unlike the standard five and a quarter inch magnetic disk, however, the three and a half inch disk has no index hole formed therein. Means other than the familiar index hole sensor are therefore required for generating index pulses.
U.S. Pat. No. 4,758,915 to Sakaguchi discloses a conventional approach to the problem of how to generate index pulses in data storage apparatus for use with the three and a half inch magnetic disk. It teaches to attach an index mark, such as a permanent magnet or optical reflector, to the rotor of a disk drive motor of the known rotor outside stator construction. An index sensor is immovably disposed opposite the orbit of the index mark, producing a pulse each time the index mark travels past the sensor. The output pulses of the index sensor serve as index pulses because the index mark on the motor rotor has a predetermined positional relationship to the eccentric drive pin on the turntable, to which the disk drive motor is coupled directly, and thence to the reference position on the disk.
An objection to this conventional approach is that the index sensor, which typically takes the form of a magnetoelectric or photoelectric converter, is not as inexpensive as can be desired. Its mounting in position adjacent the disk drive motor has also been troublesome and time consuming. Moreover, as disk drives has grown less and less in size in recent years to meet the needs and demands of users, the index sensor has come to represent a serious impediment to further reduction in the size of the disk drive.